This invention concerns metal surface treatment and in particular a process for metal surface treatment employing hydrogen peroxide in aqueous acidic solution, a solution containing hydrogen peroxide for treating metal surfaces and a stabiliser system for stabilising hydrogen peroxide.
During metal processing, a metal oxide layer is often formed over all or part of the metal surface, impairing its appearance and/or suitability for further processing steps. Thus, for example, such a problem can arise for steel, including stainless steel, titanium and its alloys, copper and its alloys and even in some circumstances for aluminium. Accordingly, it is desirable to remove or at least reduce the size and extent of the metal oxide layer. Such processes can be described as metal surface cleansing, or in some instances more particularly as metal pickling or polishing or desmutting depending on the specific treatment being carried out. In one class of processes, a solution of hydrogen peroxide is employed, and often an aqueous acidic solution to remove the oxide layer.
During the course of the metal surface treatment, the removal of the oxide layer causes metal to pass into solution. It is uneconomic and wasteful of resources to discard metal treatment solutions frequently, so that over the course of time, the concentration of metal in solution increases. The metals which pass into solution are usually the commonly employed metals such as iron, copper and metals alloyed with them such as nickel, chromium and titanium. Also in some treatment processes such as in steel pickling, it is often the metal, ie iron, in solution which carries out much of the oxidation, and accordingly a significant concentration if the metal is deliberately maintained from the start of the process. These metals in acidic solution catalyse the decomposition of hydrogen peroxide so that it is desirable to find means of minimising the rate and/or extent to which the hydrogen peroxide decomposes.
In order to retard peroxide decomposition in acidic solution, it has been proposed hitherto to include a number of different inorganic or organic substances, which can often be alternatively described as chelating agents or free radical inhibitors. Without being limited to the particular mechanism by which stabilisation, ie peroxide decomposition retardation, is achieved, a number of the stabilisers can be described chemically as alcohols (U.S. Pat. No. 3,869,401, U.S. Pat. No. 3,556,883), carboxylic acids (U.S. Pat. No. 3,537,895), phosphonic acids (U.S. Pat. No. 3,122,417 U.S. Pat. No. 4,059,678) or sulphonic acids (U.S. Pat. No. 3,801,512). Amongst the lists of stabilisers proposed for acid solutions, p-hydroxybenzoic acid has been disclosed in WO 91/5079 (Solvay Interox Limited).